The present invention relates to a process for synthesizing a large single crystal diamond by the temperature gradient method.
Synthesis of diamond by the temperature gradient method has been known in the art which was first established by the General Electric Company as described in U.S. Pat. No. 4,034,066 which is incorporated herein by reference. As a result of subsequent efforts toward large-scale production (Japanese Patent Application (OPI) Nos. 152214/84 and 210512/85), diamonds synthesized by this method are commercially available as heat sinks, superprecision cutting tools, and other fabricated products. (The term "OPI" used herein means a published unexamined Japanese patent application.)
The temperature gradient method is shown schematically in FIG. 1. A vessel (not shown) capable of producing superhigh pressures is equiped with a carbon source (1), a solvent (2), a seed crystal (3) and a seed crystal dissolution preventing layer (4), and the vessel is heated with a heater (5) to produce a differential temperature .DELTA.T between the carbon source and the seed crystal, which is used as a driving force for the epitaxial growth of a single crystal diamond on the seed crystal.
The conventional methods of diamond synthesis can produce single crystal diamonds of up to 2 carat (6 mm in diameter) but such methods cannot applicable to the production of diamonds having a diameter of 8 mm or more which are used in CO.sub.2 laser windows, infrared light emission windows, medical scalpels, IC bonders, etc. In addition, a growth period of at least two weeks is required to synthesize a diamond of 2 carat, and this is costly and is not suitable for practical applications.
These defects of the conventional diamond synthesis methods can mainly be ascribed to the following two reasons:
(i) If a large single crystal diamond is intended to synthesize from a large seed crystal (.gtoreq.3 mm), only a cluster of crystals is obtained instead of a single crystal with good quality;
(ii) If the crystal grows to 2 carat or larger, inclusions (entrapment) of the solvent into the crystal is accelerated and a desired single crystal with good quality cannot be attained.
If these problems are intended to avoid by the conventional methods, gradual growth from a small seed crystal must be conducted but the growth period is much extended so that the synthesis of diamond becomes a very costly process. Furthermore, the accelerated inclusions of the solvent (i.e., a metallic solvent) occur when the crystal size exceeds 2 carat, and prevents the synthesis of a commercially acceptable single crystal diamond.